Land drainage has long been accomplished through underground conduits or pipes formed of tile or tubing. Particularly in recent years, land drainage has been accomplished by the installation of flexible corrugated drainage tubing. In connection with such land drainage, it has long been recognized that certain soils will quickly clog drainage lines that may be installed unless provision is made for protecting the drain tile or tubing against intrusion of soil particles. Various filter materials have been proposed and have been developed for preventing such early blockage of a drainage line, including the knit fabric protective sleeves or filters described in and protected by the patents resulting from the aforementioned related applications.
It has also been discovered that certain chemical reactions occurring in soil may contribute to blockage or filling of drain lines. In particular, difficulty has occurred with a particular iron composition known in the land drainage industry as ocher. While the filter arrangements currently in use within the land drainage industry successfully deal with intrusion by fine soil particles, difficulty has upon occasion been encountered with ocher and with other similar chemical reaction related particles which may accumulate on filter surfaces or within drainage lines.